Abstract: A parenteral nutritional diagnostic system, apparatus, and method are disclosed. In an example embodiment, a parenteral nutritional diagnostic apparatus determines muscle quantity and muscle quality of a patient's psoas muscle to determine a nutritional status of the patient. An image interface is configured to receive a medical image including radiodensity data related to imaged tissue of the patient. The apparatus also includes a processor configured to use the medical image to determine a tissue surface area for each different value of radiodensity and determine a distribution of the tissue surface area for each radiodensity value. The processor is configured to determine muscle quality by locating a soft tissue peak within the distribution that corresponds to a local peak in at a region related to at least one of muscle tissue, organ tissue, and intramuscular adipose tissue. The processor determines the nutritional status of the patient based on soft tissue peak.